A diesel particulate filter (DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine. A well known DPF type is a wall-flow type filter made of cordierite (a ceramic material). The DPF is designed to force the exhaust gas from a diesel engine to flow through the wall of the filter while the particulates collect on the filter wall. Wall-flow DPF usually remove 85% or more of the soot, and can at times achieve efficiencies of close to 100%. A diesel engine equipped with functioning DPF emits no visible smoke from its exhaust pipe.
However, after a period of operation, the DPF must be regenerated (i.e., removal of collected particulates or soot from the wall of the filter). Regeneration of the DPF can be achieved by burning off the accumulated particulates, either through the use of a catalyst (i.e., passive) or through an active method such as engine management to heat the DPF to particulate/soot combustion temperature. Since the exhaust temperature of a diesel engine during normal operation is around 150-250° C., considerably lower than what is required for thermally regenerating a DPF, there is a need to increase the temperature of the DPF to around 550-850° C. to initiate a self-propagating particulate/soot combustion event. Some examples of regeneration methods include: microwave energy, resistive heating coils, fuel burning/engine management, and catalytic oxidizers. These methods generally cycle through relatively long soot/particulate accumulation times alternating with short and high temperature regeneration periods. Two of the methods rely on increasing the engine exhaust gas temperature to the point where soot/particulate combustion occurs. These methods require the use of either an electric heater or a fuel burner to directly heat the exhaust gas that in turn indirectly raises the temperature of the DPF. The downside of either approach is that not all the heat transferred to the exhaust gas is transferred to the DPF. Much of the exhaust passes through the DPF with incomplete heat transfer, creating a large inefficiency. The problem is worse for the case of the fuel burner. Here the inefficiency is compounded by the creation of additional particulates and hydrocarbon emissions, a lower exhaust oxygen concentration, and shorter lifetimes for the DPF due to cracking from thermal gradients.
Microwave heating has also been explored as a method to efficiently raise the temperature of the DPF to the soot/particulate combustion temperature. To achieve this, either the entire DPF or at least selected regions of the DPF is made of a material that is able to absorb microwave energy at the frequency of operation. This has led to strategies where the entire DPF is made from an expensive microwave absorbing ceramic material such as silicon carbide (SiC), or where a standard cordierite DPF is selectively coated with an absorbing material. In both cases, parasitic absorption of microwave energy by the soot/particulate effectively reduces the regeneration efficiency to a undesirable level.
Generally, a filter system can employ one of two different types of resistive heating methods. As to a first type, the heating element is integrated into the filter itself consisting a single or mixed element coating that is heated to regeneration temperatures. As to a second type, the filter system contains a separate heating element that is heated separately to transfer heat to the filter system to obtain the desired regeneration temperatures. However, the heating element of the filter system may suffer from changing resistivity due to changing temperature throughout a regeneration cycle.
A conventional wall-flow type DPF made from the mineral cordierite is effective in trapping diesel particulates/soot, but once the DPF is saturated, it must be regenerated to remove the trapped particulates. One method for regeneration of a DPF is by controlled combustion of the particulates initiated from an exothermic event at the front face of the DPF. FIG. 1 illustrates a resistively heated metal coil 10 in a spiral shape installed on the front face 20 of a DPF 30 which is loaded with particulates/soot. By applying an electrical power across the two terminals 35 of the metal coil 10, the metal coil 10 is heated up.
FIG. 2 illustrates how the resistively heated metal coil 10 in contact with the front face 20 of the DPF 30 can initiate the regenerative combustion process. That is, resistive heat generated from the resistively heated metal coil 10 ignites particulates/soot 40 trapped in the front section of the DPF 30. As the particulates/soot combust locally, the heat generated by the exothermic reaction ignites particulates/soot in the neighboring section of the channel within the DPF 30. The process is repeated as the particulates/soot combustion exotherm propagates along the entire length of the DPF 30. Alternative, the resistively heated metal coil 10 can be replaced with a metallic thick film coating with the same pattern on the front face 20 of the DPF. Methods of depositing a metallic coating includes, for example, chemical vapor deposition, spray coating, dip coating, and screen printing.